Traveling-wave tubes



Dec. 30, 1958 R. ADLER TRAvELING-WAVE TUBES Filed April 15, 1954 HISATTORNEY.

Asi'

TRAVELING-WAVE TUBES Robert Adler, Northfield, lll., assigner to ZenithRadio Corporation, a corporation of Delaware Application April 15, 1954,Serial No. 423,277

12 Claims. (Cl. S15-3.6)

This invention relates to new and improved electrondischarge devices ofthe traveling-wave type. More particularly, the invention is directed totraveling-wave tubes suitable for use as amplifiers operable over arelatively wide range of frequencies.

In the recent past, the Federal Communications Commission has authorizedthe construction of television broadcasting stations operating atfrequencies within the ultra-high-frequency range between 490 and 870megacycles per second. Consequently, manufacturers of televisionreceivers have found it necessary to provide terminal equipment' adaptedto receive programs transmitted within this frequency range. One of themost dilicult problems presented in the construction of a televisionreceiver of this type results from the fact that conventionalintensity-control electron tubes (triodes, pentodes, etc.) are not wellsuited for use as amplifiers within the ultra-high-frequency range; moreparticularly, it is extremely dilicult to achieve uniform gainthroughoutl the U. H'. F. range withtubes having practical dimensionsand tolerances. Accordingly, it has generally been considered preferableto apply the received' signal-directly to a hetero dyning stage withoutamplication. In this event, however, the, picture reproduced by thereceiver is often seriously disturbed by thermal noise.

One type ofelectron-discharge device which is capable of providingamplification over a relatively wide range of high frequencies is theconventional travelingwave tube. In these tubes, a radio-frequencysignal is applied to a low-velocity wave-transmission line, which, inits simplest form, may comprise a helically wound conductor.. Anelectron stream is directed along a path closely adjacent toV the'helical line; usually, the electron beam path coincides with the axis ofthe helix. The velocity ofthe electrons in the'beam is madesubstantially equal to the eiectivez velocity of the radio-frequencysignal wave traveling along the line. The electron beam isAvelocity-modulated bythe electrostatic i'eld developed by the signalwave traveling along the line, and," in turn, induces current in: theline which may amplify the radiofrequency. signal. However, suchtraveling-wave tubes are much too largeV and .expensive for use in"V atelevision receiver.

A little-known Variant of the traveling-Wave tube comprises a deviceadaptedfor push-pull or transverse mode operation, as opposed` to thelongitudinal or velocitymodulation mode of operation employed intheconventional tubes. A transverse-mode traveling-wave tube mayf compriseva pair of low-velocity wave-transmission lines mounted in substantiallyparallel spaced relationship with respect to each other. Aradio-frequencyinput` signal is applied in pushLpull relationship, to'the two wave-transmission lines, and an electron stream is projectedtalong a path intermediate'thetwo transmissionlines at avelocity-substantially equal. to theeffective propagation velocityof-fthe signal-wave alongthelength of .the lines. Consequently,eachrelect-ronofthestream 1, States Patent quency signal so thatexponential amplification is attained. t

In both conventionalv and transverse-mode travelingwave tubes, it isessential that the electron stream be conlined to a relatively narrowpath so that the electrons are not collected by the wavetransmissionlines. A magnetic field extending throughout the length `of the electronbeam path is generally employed lto confine the electrons to that pathand topre'v'ent dispersion of the beam. A relatively bulky and expensiveelectromagnetic coil surrounding the entire traveling-wave tube isusually utilized for thisV purpose. However, 'such a structure is notdesirable in apparatus such as a television receiver, where space andcost considerations are of paramount importance.

It is a primary object of the invention, therefore, to provide a new andimproved electron-.discharge device suitable for usel as an amplier overa relatively wide range of ultra-high frequencies.

It is a further object of the invention to provide an electron-dischargedevice, capable of; operating as a broad band' U. H. F. amplilier, whichisrelatively small in size but which provides an acceptable degree ofamplification.

It is a specific object of the invention to providev a new and` improvedelectron-discharge device of the transverse-mode traveling-wave type inwhich the electron stream is effectively confined to a predetermined"path without requiring the use ofl a magnetic collimating system.

It is another object of the inventionY to provide a new and improvedtransvers'emode,traveling-wave tube in which thermal noise issubstantially minimized.

It is a corollary object of the invention to provide a traveling-wavetube which is relatively simple and expedient to construct andeconomical to manufacture.

An electron-discharge'device constructed in accordance with one aspectofthe invention comprisesr an electron gun for projecting a sheet-likebeam of electrons along a given reference path. A wave-transmission lineis disposed adjacent to the reference path and'is electrostaticallycoupled to the electron beam; this wave-transmission line has a lowwavepropagation velocity in a direction parallel kto the reference pathand further has a length in that direction which is large relative tothevr effective wavelength of a signal wave. traveling along the line.

A support member isinterposed between thewave-trans-V mission line andythe reference path,and a first series of electricallyinterconnectedelectric signal field permeable' conductive elements taredisposed at predetermined intervalsralongthe reference pathon thesurface of the support member adjacentthat path; Afsecond series ofelectricallyirterconnected electricv signal field permeable theinvention comprises-'anlelectro"gun'for projecting' a sheet-likebeamofelectrons alonga given reference path.Y A-pair ofbe'am-conliningstructures, each com` prisng aplurality'ollr series of lens electrodes,arev disposed on opposite sides of the reference path and are employedto confine the electron beam to that path; these beam-confiningstructures are permeable to signal-frequency electric fields but aresubstantially impermeable to unidirectional electric fields. Awave-transmission line is disposed adjacent to the reference path but isseparated therefrom by one of the beam-confining structures, so that thetransmission line is electrostatically coupled for signal frequenciesonly to the electron beam. This wave-transmission line has a lowwave-propagation velocity in a direction parallel to the reference pathand further has a length in that direction which is large relative tothe effective wavelength of a signal wave traveling along thetransmission line.

The features of the invention which are believed to be novel are setforth with particularity in the appended claims. The organization andmanner of operation of the invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals refer to like elements in the severalfigures, and in which:

Figurel is a cross-sectional view, partially schematic, of oneembodiment of an electron-discharge device constructed in accordancewith the invention, and includes a schematic representation of asimplified amplifier circuit;

Figure la is an explanatory diagram illustrating certain operationalfeatures of the apparatus of Figure 1;

Figure 2 is a cross-sectional view taken along line 2--2 of Figure 1;

Figure 3 is a fragmentary cross-sectional view taken along line 3-3 inFigure 2; and

Figure 4 illustrates an alternative construction for that portion of theinvention illustrated in Figure 3.

The embodiment of the invention shown in Figure 1 comprises anelectron-discharge device or traveling-wave tube 20; tube 20 includes acathode 10 having an electronemissive surface 11. A focusing electrode12 is mounted in parallel spaced relation to surface 11 and includes acentrally located slot 13. An accelerator electrode 14 including anaperture `16 is included in device 20 and is positioned adjacentfocusing electrode 12 with slot 16 opposite slot 13. A beam-limitingelectrode 15 is mounted in spaced relationship to accelerator 14 on theside of the accelerator opposite electrode 12; electrode 15 includes abeam-limiting aperture 17 aligned with aperture 16. Cathode andelectrodes 12, 14 and 15` comprises an electron gun 19 for projecting anelectron beam along a reference path indicated by a dash line Acomprising the center plane of the path;' reference path A terminates ata collector electrode 18 positioned at the opposite end of tube 20.

A first low-velocity wave-transmission line 21 is dis- I posed adjacentreference path A intermediate electrode and collector 18. Transmissionline 21 comprises a helical conductive winding 22 wound upon alongitudinal support member 24; support member 24 is preferably formedfrom some insulating material such as glass or ceramic materialsuitablefor use in a vacuum tube. It should be understood that the sizeof conductive winding 22 has` been exaggerated in Figure l in order tofacilitate the presentation of the inventive concept, and that only arelatively few turns of the winding are illustrated as compared to thenumber which may be employed in practice. t j

Tube `also includes a second low-'velocity wavetransmission line 25which is disposed adjacent reference path A on the opposite side of thepath from line 21. Line 25, which is essentially identical in electricaland physical characteristics with line 21,` co`mprises.a helicalconductive `winding 26 mounted on an insulating `support member 27. Thelength Z of wave-transmission lines 21 and 25 determines theiportion ofpath A in which interaction `between the electron beam and a signal wavetraveling along lines 21 and 25 may occur; length Z is relatively largeas compared to the effective wavelength of a signal wave traveling alongthe transmission lines.

Preferably, the electron beam developed by gun 19 is sheet-like in form;in other words, the electron beam has one principal crosssectionaldimension which is very much greater than a second principalcross-sectional dimension, these dimensions being generally determinedby the configuration of slot 17. A beam of this type is normally quiteadvantageous in a weak-signal amplifier, due to the fact that it permitsrealization of increased amplification from a tube of given maximumdimensions.

As seen in the cross-sectional view of Figure 2, the thickness of theelectron beam, which is determined by dimension t of slot 17, is verymuch smaller than its height, which corresponds to slot dimension h, sothat the beam has a cross-sectional configuration corresponding to anelongated reactangle. It should be understood that although arectangular cross-sectional configuration has been illustrated, otherbeam configurations may be employed. Support member 24, upon whichwinding 22 of line 21 is wound, is essentially I-shaped incrosssectional configuration, and the longitudinal support member 27which carries winding 26 of line 25 is similarly shaped.

A pair of beam-confining structures 28 and 29 are included within tube20; beam-confining structure 28 is interposed between wave-transmissionline 21 and refcrence path A, Whereas structure 29 is interposed bctweenline 25 and path A. Thus, each of the wave-trans-v mission lines isdisposed adjacent to the reference path but is separated therefrom byone of the beam-confining structures. Beam-confining structure 28comprises a longitudinal support member 30 which may be formed from micaor other suitable insulating material; the support member is preferablymade as thin as possible consistent with adequate structural strength.As best shown in Figure 3, a first series of electrically interconnectedconductive elements or lens electrodes 31 are disposed at predeterminedintervals along the surface of support member 30 adjacent reference pathA (Figure l). Conductive elements 31 have a relatively high resistivityso that they do not comprise a unipotential plane or shield at radiofrequencies; preferably, electrodes 31 are formed as thin highlyresistive coatings of carbon or other conductive material deposited onthe surface of support member 30. For example, elements 31 may eachcomprise a coating having a resistivity of the order of ohms per square,A second series of electrically interconnected conductive elements 32are formed on the same surface of support member 30 as conductiveelements 31; lens electrodes 32 are interleaved with elements 31 and arepreferably substantially identical in physical and electricalcharacteristics with the first series of lens electrodes.

Beam-confining structure 29 is essentially identical in constructionwith structure 28 and comprises a thin insu lating support member 33. Afirst series of conductive elements or lens electrodes 35 are disposedalong the surface of support member 33 adjacent beam path A directlyopposite lens electrodes 31 of structure 28. A second series ofinterconnected conductive elements 36 are interleaved with lenselectrodes 35 and are disposed di rectly opposite conductive elements 32of structure 28. In addition, those portions of the surfaces of supportmembers 30 and 33 adjacent beam path A not covered by the lens electrodecoatings may be provided with a leakage coating 39 of conductivematerial having a substantially higher resistivity than the lenselectrodes in order to avoid charging the exposed insulator surface byinavoidable stray electrons; for example, the resistivity of coating 39may be of the order of 10a ohms per square. Alternatively, a completeconductive coating of similarly high resistivity may be applied acrossthe entire surassenzio' face of the insulating support members and thelens electrodes, or the support members themselves maybe cons tructedfrom `a"material having a'suitableI amountcf volume o'r'surfaceresistivity. i

Tube 20 may `be` provided with a suitable base for envelope 29'and yaseparate 'heater larnent may be included in cathode because thesestructural details are familiar in the art, they are not illustrated inthe drawings. Envelope 29 may be of conventional receiving-tube size.After wave-transmission lines 21 land 25, collector 18, and theelectrodesv comprising electron gun 19 have been mountedwithin envelope29, the envelope is evacuated and gettered in an'y manner known in theart.

A simplified amplifier circuit for tube 20 has been schematicallyillustratedin Figure lin'orde to facilitate the description oftheoperatin of the tube. A balanced signal source 37 is connectedbetween the ends of winding 22 of line 21 and 4winding 26 of line 25adjacent electron gun 10. Source 37 may comprise any suitable source ofradio-frequency signals, and may, for instance, constitute thetermination of a balanced television antenna. A balanced lad circuit 38is connected between the ends ofk windings 22. and 26 of transmissionlines 21 and 25 adjacent collector 1S.

Cathode 10 is connected to a plane of reference potential, hereillustrated as ground, and focusing electrode 12 may also be connectedtoground; if preferred, however, when the amplifier is incorporated in atelevision receiver or the like, electrode 12 may be connected to asuitable source of automaticy gain control potential (not s hown) tomaintain a relatively constant output signal amplitude. Accelerator 14is connected t-o a first source of positive unidirectional operatingpotential B1-}-, and electrode 15 is connected to a second positivevoltage source B2+. A third source of po-sitive potential, B34-, isconnected to conductive coatings 31 and 35 of beam-conlining structures28 and`29 respectively; similarly, a fourth D; C. voltage sourceB4-l-'is connected toV conductive coatings 32 `andi36 of thebeam-confining structures. Collector 18 is electrically connected to anadditional source of positive operating potential B54-, It will beunderstood, of course, that all of the B-lvoltage sources may compriseseparate taps on a single unidirectional or D. C. voltage source.Furthermore, some of the B+ potentials (e. g. B1+ andB5-}) may be of thesame value.

Intraveling-wave-tubes constructed in accordance with known techniques,common experience indicates that the use of electrodes which interceptany appreciableportion of the electronbeam may produce highlyundesirable partition-noise effects; accordingly, the electron guns ofconventional tubes employ electrode structures in which an attempt ismade to avoid interception, by the gun electrodes, of any substantialportion of the beam. One percent interception, for instance, isgenerally considered excessive. However, it has been found, suprisinglyenough, that wthese partition-noise effects are not' significant intransverse-mode traveling-wave tubes; indeed, electron guns includingelectrodes which intercept fifty percent or more of the total beamcurrent have been found to provide inherently better noisecharacteristics, in transverse-mode tubes, than electron guns of thetype utilized in conventional tubes. Consequently, electrode 15ispreferably constructed to intercept a substantial portion of the totalbeamcurrent, preferably greater than fifty-percent.

When vdevice 20 is placed in operation, a radio-frequency signal isapplied to wave-transmission lines 21 and 25 in push-pull relationship,from source 37. Due to the helical configuration of the windings whichform the wavetransmission lines, each of lines 21 1and'25 has awave-propagation velocity in a direction parallel to reference path Awhich is considerably smaller than the propagation velocity o felectromagnetic radiation in free space. The actualeffectivewavepropagation velocity of the lines is a matter of designchoice and may be as low as one one-hundredth (0.01 of the free-spacepropagation velocity. Electrons emitted from surface 11 of cathode 10are focused by passing through slot 13 of electrode 12 and areaccelerated as they traverse slot 16 of accelerator 14, due to thepositive operating potential applied to the accelerator from sourceB1-|. Part yof the electron stream then passes through aperture 17 ofelectrode 15, which may be held at a potential somewhat below that ofaccelerator 14 but positive with respect to the cathode. Preferably, theaverage potentials supplied to the conductive elements 31 and 35 shouldbe different from that applied to electrode 15 from source B2-{-, sothat an electrostatic focusing lens is formed between electrode 15 andlens electrodes 31a and 35a. The electrons of the beam continue alongreference path A and are collected by electrode 18. The beam velocity isdetermined by the average of the D. C. potentials of conductive elements31, 35 and 32, 36 with respect to cathode 10.

In order to reach a more complete understanding of the advantages anduseful qualities of the invention, the arnplier illustrated in Figures land 2 may first be consideredv as operating without the benefit of anycollimating or focusing field tending to confine the electron beamv tothe maximum width d 0f reference path A; that is, beam-confiningstructures 28 and 29 areconsidered to be omitted. The velocity of theelectron beam along path A may then be adjusted so that it isapproximately equal to the wave-propagation velocity of the signal wavetraveling along transmission lines 21 and 25. Each electron or group ofelectrons instantaneously emerging into the portion of reference path Adefined by transmission line length Z is subjected to a transverseelectric eld established by the application of radio-frequency signalsfrom source 37, in phase opposition, to the two transmission lines.Because the electron `and wave-propagation velocities are equal, eachindividual electron is continuously deected in a given direction andbegins to move transversely with respect to path A as well as parallelthereto. As the electrons move away from the center plane of thereference path, they form a wave pattern which, by moving along thewave-transmission lines, induces a signal current in the lines; thisinduced current is out of phase with respect to the original fieldsupplied by source 37.

Interaction between the traveling-wave` field and the electron streamleads to the emergence of three .separate waves in place of the originalsignal wave; only one of the three, waves grows exponentially as ittravels along the transmission lines. This process has beenanalyzed inchapter 13- of the book entitled Traveling-WaveTubes by I. R. Pierce,published by D. Van Nostrand'Co., Inc. New York, 1950.

Gperation of `theamplilierV of Figures l-and V2'under the conditionsjust described might be highly successful if the electrons projectedfrom emissive surface 11 of cathode 10 all followed paths exactlyparallel to reference path A and if space-charge effects could beignored. In practice, however, this is not possible. On the average,each of the electrons emerging from electronfgun 19 has some initialtransverse velocity which causes the beam todispersel and,` in addition,space-charge effects tend further to spread the beam. Consequently, ifno provisions are made for focusing or collimating the electron beam asit traverses reference path lengthZ,it is extremely difiicult to secureany appreciable Ygain from traveling-wave tube 20, since theelectronrbeamrapidlydispersesand is collected by the beam` confiningstructures. Conventionally, magnetic collimating fields have beenemployed to restrict the transverse excursions of the beam electrons;the structuresemployed todevelop the magnetic collimating field,however, are 'consideredexcessively bulky and expensive forl domestictelevision receivers and similarV applications.

Traveling-,wave-tube 2f?, onthe other hand, includes electrostaticfocusing system, comprising structures 28 and 29, for confining theelectron beam within maximum width d of reference path A; width d islimited by and must be smaller than the spacing between the twobeamconfining structures. Lens electrode series 31 of beamconfiningstructure 28 is maintained at the same potential as conductive elementseries 35 of structure 29. Lens electrode series 32 and 36, on the otherhand, are maintained at a different D. C. potential, so that there is asubstantial potential difference between the first set of' lenselectrodes, elements 31 and 35a, and the next set of lens electrodes 32aand 36a. Thus, throughout interaction space length Z, each set ofopposed lens electrodes is maintained at a different D. C. potentialfrom the adjacent sets of conductive elements. This condition isillustrated in Figure 1a, which comprises a schematic representation ofthe lens electrodes; the relative sizes and spacings of the elementsillustrated therein have been distorted somewhat in order to assist inexplaining the figure and to provide more space. resulting from thesignal wave traveling along lines 21 and 25 (Figure l) is relativelysmall in comparison with the difference in D. C. potential between theindividual windings of each line.

As shown in Figure la, an electron entering the portion of referencepath A bounded by the wave-transmission lines may have a velocitycomponent in the transverse direction indicated by arrows y as well as aprincipal velocity component parallel to reference path A. lThetransverse velocity component may result from thermal or space-chargeeffects or other factors. The electron may enter the interaction spacebounded by lens electrodes 31, 32, 35 and 36 along a hypothetical pathA', and, at the outset, is subjected to an electrostatic field primarilydetermined by the average or steady-state potential applied to the firstset of conductive elements 31a and 35a from source B34- (Figure l). Asthe electron continues along path A', it reaches the space betweenelements 31a, 35a and the next set of lens electrodes, designated 32a,36a. Because elements 31a and 35a are connected to source 133+ and aremaintained at a considerably different potential from lens electrodes32a, 36a, the electron encounters a convergent electron lens actionwhich tends to deflect the electron toward center plane A of thereference path. T'he electron continues along path A', and, uoonreaching the space bounded by conductive coatings 32a, 36a and 31b, 35h,enters another convergent electrostatic lens. Consequently, the electronis again deflected toward center plane `A. Thus, as the electronproceeds along path A it is subjected to a periodic electrostatic lensfield effectively constituting a series of convergent electrostaticlenses. The periodic lens field tends to deflect the electron towardreference path center plane A with a force which is proportional to thedisplacement of the electron from that plane; accordingly, the lensfield is generally equivalent to a transverse elastic field and confinesthe electrons `of the'beam to maximum path width d. y

The focal length `ofeach of the electrostatic lenses formed betweenadjacent sets of lens electrodes, such as the lens between elements 31a,35a and 32a, 36a, is proportional to the transverse spacing betweenthose sets of conductive elements and is a function of the ratio betweenthe D. C. potentials of the sets of lens electrodes with respect tocathode 10. Thefocal lengths of the electrostatic lenses, in conjunctionwith the spacing s between adiacent lenses, determines the distancealong path A which is traversed by an electron having a given initialtransverse velocity before that electron crosses the center plane ofthereference path. Consequently, as shown by trajectoryA, each electron(other than those having no initial transverse velocity) followsasubstantially sinusoidal'path which is symmetrical with respect toreferencepath center plane A. A second hypothetical electron Thesignal-frequency potential path A" illustrates the trajectory of anelectron having a different initial transverse velocity and a differentstarting position from the electron following path A'. As indicated bytrajectory A, the magnitude and direction of the initial transversevelocity and the original displacev ment of the electron with respect tocenter plane A do not affect the wavelength Le of the sinusoidal pathsfollowed by the individual electrons.

For a given structure, wavelength Le is determined by the strength ofthe lens field produced by the D. C. po tential difference betweenadjacent sets of lens electrodes, and by the average of their individualD. C. potentials which establishes the average velocity of the electronstream. As each electron follows its individual trajectory, it carriesout a transverse harmonic motion at a frequency we equal to its averagevelocity divided by wavelength Le. This transverse motion is analogousto the motion of a mechanical resonator such as a vibrating reed; aperiodic force having a frequency equal to the natural frequency of sucha resonator produces a periodic motion of linearly increasing amplitude.Consequently, the electron stream may be said to exhibit a transverseresonance at the frequency we.

The frequency of the signal applied from source 33 may be designated woand the propagation velocity of the undisturbed signal wave travelingalong the lines may be taken as v0. The average velocity of theelectrons may be designated ve.

By proper choice of the D. C. potentials applied to thewave-transmission lines, the velocity of the electron beam may beadjusted so that Under these conditions, the individual electrons of thebeam are subjected to a periodically changing signal field having afrequency we; because the electrons tend to resonate at that frequency,they begin to move at increasing amplitudes in the y direction. Itshould be recalled that the electrostatic lens field is relativelystrong in comparison to the signal wave field, so that the transverseexcursions of the electrons induced by the signal field do not cause theelectrons to be collected by the wave-transmission lines.

Under the conditions described immediately above, the current induced inwave-transmission lines 21 and 25 by the pattern of transverselyvibrating electrons moving along path A is in phase with the signal Waveapplied from source 37, so that gain is achieved. The amplitude of thesignal wave, at any point along the lines, may be eX- pressed as existalong wave-transmission lines 21 and 25; this has been determined to betrue, The fact that the initially applied signal is effectively dividedinto these two waves accounts for the factor 1/2 inthe above equation.For a tube which is long enough to afford substantial gain, theattenuated wave is of negligible effect as compared to the amplifiedwave, so that a simplified expression for the gain of such a long tubeis (3) nel/2meZ This compares quite favorably with the conventionalveexpression for the growth constant indicates division of the appliedsignal into three waves, so that the equation for a long tube includes afactor of only 1/3 instead of 1/2.

It should be noted that the phase conditionswithn tube 20 whichresult inthe direct addition of the induced current tothe original signal currentin wave-transmission lines 21 and 25 prevail accurately only so long asthe amplitude of the driving or signal field does not change along thelength of the wave-transmission lines. Actually, the driving fieldincreasescontinuously, so that a phase error occurs. This phase error,yhowever, is relatively small and may be readily eliminated by minoradjustments in either the transverseresonance frequency orthe electronbeam velocity; such adjustments may conveniently be made by varying theapplied potentials from one or more of the D. C. sources B34- or 134+.

In order to achieve effective operation of tube 20, several conditionsshould be met. ,To avoid difficulties presented by lens aberrations, thespacing s between adjacent lenses (Figure la) should be greater thanthree times the maximum permissible width d of reference path A (theproportions illustrated in yFigures l and la-are at variance with thiscondition to avoidV overcrowding). The effective wavelength Le of thelens field (Figure la) should be equal to or greater than three timeslens spacing s. Furthermore, transmission-line length Z must be large inrelation to the effective wavelength of the signal wave as it travelsalong the wave-transmission lines; this latter condition must be met inorder to achieve appreciable gain. Support members 30 and 33 and theconductive coatings comprising lens electrodes 3l, 32, 35 and 36 must,of course, be thin enough and have sufficiently high resistance so as tohave only a small effect upon the signal frequency electric fields, inorder that the wavetransmission lines may be effectively coupled to theelectron beam. The resistance of conductive coatings 3l, 32, 35 and 36is made low enough so that the D. C. potential of the lens electrodesremains substantially uniform through each lens electrode surface andrelatively close to the desired value despite the unavoidable voltagedrop produced by stray electrons fro-m the beam impinging upon the lenselectrodes. Because the lens coatings are positioned within thesignal-frequency field, reduced gain results if the resistance ofelectrodes 31, 32, 35 and 36 is made too low; a certain amount of suchloss may be desirable, however, in order to increase the operatingstability of the device, in accordance with techniques well known in thetraveling wave tube art. i

The electrostatic lenses established along path A determine thetransverse resonant frequency of the electron beam and, at the sametime, confine the beam within width d so that it does not impinge uponthe wave-transmission lines. Consequently, it is possible to make lines21 and 25 suiciently long to achieve useful gain from tube .20. Theelectrostatic. lens field centers the beam about center plane A, asVcontrasted to the mere collimating action of a-conventional magneticfield, so that width d -may be held to avminimum. This facilitates closecoupling between the electron beam and the wave-transmissionlineszandpermits the realizationof greater amplification ina tube of givenoverall size.

i It has beenrdeterrnined that the exponential gain or growth constant aof a transverse-mode traveling-wave tube such as device 20 is vadverselyaffected by stray capacities valong the'wave-transmission lines. Inorder to minimize such stray capacities, support members 24 and 27preferably have a transverse cross-sectional area which is.. as` smallas possible; .in addition,- the material from which the supports aremade should have a low dielectric constant.` The-Ishaped`cross-sectional configuration for the ksupport members illustrated inFigure -2 is advantageous in this respect -in thatl it presents arelatively small cross-sectional areawhich is widely separated from theindividual turns... Support-members 24 and 27 may be Iformed fromceramic, glass, or other dielectric materials adapted for use in avacuum. Helix windings 22 and-'26',

of course,` may be formed from copper, molybdenum,-or other conductivematerial suitable for'use in a vacuum; In the embodiment ofthe'invention illustrated in Figures 1 and-2, the electrostatic lensfield utilized to confine the electron beam Within Width d isestablished solely by the D. C. potential between the individual sets oflens electrodes of beam-confining structures 28 and 29, which shield theelectron beam from any zero frequency or unipotential electric fielddeveloped by the transmission lines, although they are effective incoupling to the beam the signal-frequency field 'created by the signalwave traveling along transmission lines 21 and 25. It is possible toconstruct devices in which portions of the wave-transmission linesthemselves function as lens electrodes and'in which thewave-transmission lines may be coupled to the electron beam by means ofseparately identifiable lens electrodes which form a part of the lines.Structures of this general type are described in the copendingapplications of Robert Adler, Serial Nos. 394,797 and 394,798, bothfiled November 27, 1953. Moreover, periodic electrostatic lensstructures may also be advantageously employed intraveling-wave tubesincluding only a single wave-transmission line and specifically adaptedfor longitudinal-mode operation, as described and claimed in thecopending application of Robert Adler, Serial No. 401,149, filedDecember 30, 1953; all of these applications are assigned to the sameassignee as the present invention.

Figure 4 illustrates another type of beam-confining structure which maybe employed in place of structures 28 and 29 of Figures 1 3. As shown inFigure 4,`the beam-confining structure 43 may comprise a relatively thininsulating support member 49 substantially similar to thepreviously-described members l3A0-and 33. Support member 49 may beprovided with a very highly resistive leakage coating (not shown) topreclude the collection of undesirable charges on the' support membersurface.

Beam-conning structure 48 further includes a first series of lenselectrodes 56 each includingl aplurality of thin, highly conductivestrips --51 extending across the surface of support member 43transversely with respectv to the direction of beam travel, generallyindicated yby arrow K. Conductive strips 51 may, for example, comprisethin lines of silver printed or otherwise deposited upon support member49. Each'of strips 51' is electrically connected to a conductive member52 which has a high impedance at signal frequencies; member 52Hm'aycomprise a'radio-frequency choke or a resistor or any combination ofresistance and inductance having a relatively high impedance at signalfrequencies.

Structure 48 further includes a Vsecond series of lens electrodes 53which are substantially similar to electrodes 50; lens electrodes `53leach comprise a plurality of thin conductive strips 54Vwhich areelectrically connected 'to a second conductive member 55 having arelatively high impedance at signal frequencies. Impedance members 52and 55 are electrically connected to D. C.` operating potential sourcesB34- and BHL, sol that lens electrode strips 5l are maintained at agiven constant D. C. potential and electrode strips 54 are maintained ata-different constant positive potential.

It will be readily apparent that the structure illustrated in Figure 4may be substituted for beam-confining structure 28 and that a similarstructure may be substituted for structure 29 in tube-2 4) (Figure l)-Without effecting any substantial change in the operation of the tube.Because members 52 and-55 have a relatively high impedanceat signalfrequencies, they cannot'constitute'a short circuit for thesignal-frequency electric fields and thus do not adversely-affectoperation of the 'device'.

Moreover, because vstrips 51 and 54 do not extend 'for any substantialdistance in the direction of beam travel and wave propogation (arrow K)they cannot'constitute equipotent'ial planes extending in this directionandl therefore do not interfere with the normal operation of the tube.Although the structure of Figure 4 requires a greater number ofindividual elements for each of the lens electrodes, the necessity ofusing highly resistive materials for the lens electrode coatings isobviated and it is therefore somewhat easier to obtain uniform operatingcharacteristics for the lens electrodes.

Traveling-wave tubes constructed in accordance with the invention may bemade to provide relatively constant amplification characteristicsthroughout a broad band of frequencies; more specifically, theseamplifiers may be constructed to provide substantially constantamplification throughout the U. H. F. television range of frequencies,The tubes do not require the bulky and heavy external magneticstructures utilized in prior art devices to form magnetic collimatingfields for the electron beams; consequently, tubes constructed inaccordance with the invention may be made relatively small and are wellsuited for mounting in a domestic television receiver or similar device.Moreover, because the electrostatic lens system of the inventioneffectively centers the electron stream about a center plane rather thanmerely collimating the electrons to constrain them to paths parallel tosuch a plane, it is inherently more effective than the prior artmagnetic systems in restricting dispersion of the electron streamattributable to thermal and/or space charge effects. The operatingvoltages required are relatively low and do not unduly burden the powersupply of a television receiver; all of the electrode structures arerelatively simple in form and may be readily constructed by knownmethods, so that the tubes are not unduly expensive if manufactured on amass production basis.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may be madewithout departing from the invention in its broader aspects. The aim ofthe appended claims, therefore, is to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

I claim:

`1.\An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheetlike beam of electrons along agiven reference path; a wave-transmission line disposed adjacent to saidreference path and elcctrostatically coupled to said electron beamsubstantially uniformly throughout the length of said line, saidwave-transmission line having a uniform low wave-propagation velocity ina direction parallel to said path and further having a length in saiddirection which is large relative to the effective wavelength of asignal wave traveling along said line; a support member interposedbetween said wave-transmission line and said reference path; a r'stseries of electrically interconnected electric signal field permeableconductive elements disposed at predetermined intervals along saidreference path on the surface of said support member adjacent saidreference path; and a second series of electrically interconnectedelectric signal field permeable conductive elements disposed on saidsurface of said support member and interleaved with said first series.

2. An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheetlike beam of electrons along agiven reference path; a pair of wave-transmission lines disposedadjacent to opposite sides of said reference path and electrostaticallycoupled to said electron beam substantially uniform throughout thelength of said lines, said wave-transmission lines each having a uniformlow wave-propagation velocity in a direction parallel to said path andfurther having a length in said direction which is large relative to theeffective wavelentgh of a signal wave traveling along said line;` al`first support member interposed `between one of said wave-transmissionlines and said reference path; a first series of electricallyinterconnected electric signal field permeable conductive elementsdisposed at predetermined intervals along said reference path on thesurface of` said support member adjacent said reference path; a secondseries of electrically interconnected electric signal field permeableconductive elements disposed on said surface of said support member andinterleaved with said first series; a second support member interposedbetween the other of said wave-transmission lines and said referencepath; a third series of electrically interconnected electric signalfield permeable conductive elements disposed opposite said first serieson the surface of said second support member adjacent said referencepath; and a fourth series of electrically interconnected electric signalfield permeable conductive elements disposed opposite said second serieson said surface of said second support member.

3. An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheetlike beam of electrons along agiven reference path; a pair of wave-transmission lines disposedadjacent to opposite sides of said reference path and electrostaticallycoupled to said electron beam, said wave-transmission lines each havinga l-ow wave-propagation velocity in a direction parallel to said pathand further having a length in said direction which is large relative tothe effective wavelength of a signal wave traveling along said line; afirst insulating support member interposed between one of saidwave-transmission lines and said reference path; a first series ofelectrically interconnected high-resistivity coatings periodicallydisposed along said reference path on the surface of said support memberadjacent said reference path; a second series of electricallyinterconnected high-resistivity coatings disposed on said surface ofsaid support member and interleaved with said first series; a secondinsulating support member interposed between the other of saidwave-transmission lines and said reference path; a third series ofelectrically interconnected highresistivity coatings disposed oppositesaid first series on the surface of said second support member adjacentsaid reference path; and a fourth series of electrically interconnectedhigh-resistivity coatings disposed opposite said second series on saidsurface of said second support member.

4. An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheetlike beam of electrons along agiven reference path; a pair of wave-transmission lines disposedadjacent to opposite sides of said reference path and electrostaticallycoupled to said electron beam, said wave-transmission lines each havinga low wave-propagation velocity in a direction parallel to said path andfurther having a length in said direction which is large relative to theeffective wavelength of a signal wave traveling along said line; a firstinsulating support member interposed between one of saidwave-transmission lines and said reference path; a first series ofelectrically interconnected conductive elements disposed atpredetermined intervals along said reference path on the surface of saidsupport member adjacent said reference path; a second series ofelectrically interconnected conductive elements disposed on said surfaceof said support member and interleaved with said first series; a firstconductive leakage coating, having a substantially higher resistivitythan said conductive elements, covering all of said surface of saidsecond support member exposed to said beam; a second insulating supportmember interposed between the other of said wave-transmission lines andsaid reference path; a third series of electrically interconnectedconductive elements e disposed opposite said first series on the surfaceof said second support member adjacent said reference path; a fourthseries of electrically interconnected conductive elements `disposedopposite said second series on said surface of said second supportmember; and a second conductive leakage coating, having a substantiallyhigher resistivity than said conductive elements, covering all of saidsurface of said secondA support member exposed to said beam. l n

5. An electron-discharge dcviceof'the traieling-wave type comprising: anelectron `gun for projectingpasheetlike beam of electrons alonga givenreferen e Vpath; a pair of wave-transmission lines' disposed` adjacent jto oppositesides of said reference path'and elecitrostatic'ally coupledto said electron beam, said y wave-transmi'ssion lines each having a lowwave-propagationvelocity in a direction parallel to said path andfurther Ahaving a length in said direction which'is'; large `relativetol the effective wavelength of a signal wave traveling alongsaid line;a first support member interposed between one of said wave-transmissionlines and said referencefpath; a ijrst series of electricallyinterconnected conductive elements disposed at predetermined intervalsalong said reference path on the `surface of said support memberadjacent said reference path; a V'second yseries of electricallyinterconnected conductive elements disposed on said surface of saidsupport member and interleaved with), said first series; a secondsupport'niembery interposed between the other of saidWave-transmissionflines and'said reference path; a third series ofelectricallyinterconnected conductive elements disposed oppositesaidfi'rst" series at corresponding predetermined intervals along saidreference path on the surface of said second support member adjacentsaid reference path; a fourth series of electrically interconnectedconductive elements disposed'opposite said second series on said surfaceof said second support member; means for maintaining said rst and thirdseries of conductive elements at a first predetermined averagepotential; and means for `maintaining `said second and fourth series ofconductive elements at a second predetermined average potentialdifferent from said first potential to establish a series of electronlenses along said path and substantially confine said beam `'to saidpath throughout said' lengths of said wave-transmission'-lines. 6. Anelectron-discharge device of the traveling-wave type comprising: anelectron gun for projecting a sheetlike beam of electronsalo'ng a givenreference path; a pair of wave-transmission.lines disposed adjacent toopposite sides of ,said referencel path and electrostatically coupled tosaid electronbearn, saidVwave-transmission lines each having a lowwave-propagation velocity in a direction parallel to said path andfurther havingV a length in said direction which is large relative tothe effective wavelength of a signal wave traveling along saidwlineg afirst support member interposed'between one of said wave-transmissionlines and said reference path; a rst series ofV electricallyinterconnected conductive'elernents, each comprising al plurality ofconductive strips extending in a direction transverseV to thedirectionof travel of said electron beam, disposed at predetermined intervalsalong said reference path on the surface of said support member adjacentsaid reference path; a second series of electrically interconnectedconductive elements, each comprising a plurality of conductive stripsextending in a direction transverse to the direction of travel of saidelectron beam, disposed on said surface of said support member andinterleaved with said first series; a second support member interposedbetween the other of said wave-transmission lines and said referencepath; a third series of electrically interconnected conductive elementsdisposed opposite said first series at corresponding predeterminedintervals along said reference path on the surface of said secondsupport member adjacent said reference path; and a fourth series ofelectrically interconnected conductive elements disposed opposite saidsecond series on said surface of said second support member. 7. Anelectron-discharge device of the traveling-wave type comprising: anelectron gun for projecting a sheetlike beam of electrons along a givenreference path; a pair of wave-transmission lines disposed adjacent toopposite sides of said reference path and electrostatically coupled tosaid electron beam, said wave-transmission lines each having a lowwave-propagation velocity in a direction parallel to said path andfurther having a length inA saiddirection which is large relative totheeffective wavelengtho'ffa signal wave traveling along said line; a firstsupport member interposed between one of said wave-transmission linesand said `reference path; a first series of conductive elements, eachcomprising a plurality of conductive strips extending in a directiontransverse to the directionA of travel of said electron beamdisposed atpredetermined intervals along said reference path .on the surface ofsaid support membertadjacent said reference path; a second series ofconductive elements, each comprising a plurality of conductive stripsextending in a direction transverse to Vthe directionof travelof saidelectron beam, vdisposed on said surface of said support mem ber andinterleaved with said first series; arsecond support member interposedbetween the other of said wavetransmission lines and said referencepath; a third series of conductive elements disposed opposite said firstseries the surface `of said second support member adjacent saidreference path; a fourth series of conductive elements Ldisposedop'posite said secondtseries, on said surface of said second supportmember; and connecting means, having a relatively high impedance atrsignal frequencies, for electrically Vinterconnectingvvith respect tolower frequencies each of said conductive elements of each of said fourseries with the remaining elements of said series.

8. A n electron-dischargedevice ofthe traveling-wave type comprising.:an electron gun for projecting a sheetlike beam of electrons along agiven reference path; a pair of wave-transmission lines disposedadjacent to opposite sides of s aidI reference path andAelectrostatically coupled to said electron beam, saidwave-transmissionlines each having a low wave-propagation velocity in a directionparallel to said path and further having a length in said directionwhich is largev relative tothe effective wavelength of arv signal wavetraveling, alongtsaid line; a first' support Vmemberinterposed betweenLone lof Ysaid wave-transmission lines and saidtreference path; a rstseries of conductive elements, each comprising a plurality of conductivestrips extending in adirection transverse to the direction of travel, of,saidtelectron beam,k disposed at predetermined intervals along saidreference pathon the surface" of said support member adjacentsaidreference path; a second series ,of conductiveelements,\each comprisinga plurality of` conductive strips extending in a direction transversetothe directiontof travel of said electron beam, disposed on/saidsurfaceofgsaid support member and interleaved with said firstseries; a secondsupport member interposed between .the wother of said wave-,transmissionlines andrsaid` reference path; a third series of conductive elementsdisposedopposite said first series the surface of ,said second supportmember adjacent said reference path; a fourth series of conductiveelements disposed opposite said second series onv said surface of saidsecond support member; and four inductive connector members, each havinga relatively high impedance at signal frequencies, for electricallyinterconnecting each of said conductive elements of each of said fourseries with the remaining elements of said series.

9. An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheetlike beam of electrons along apredetermined reference path; a wave-transmission line, disposedadjacent said reference path in spaced relation thereto andelectrostatically coupled to said beam, having a uniform lowwavepropagation velocity in a direction parallel to said path andfurther having a length in said direction which is large relative to theeffective wave length of a radiofrequency signal wave traveling alongsaid line; means for applying a radio-frequency signal to saidwave-transmission line to establish said traveling signal wave andthereby to provide a traveling electric signal field which propagatesalong said reference path at said low wavepropagation velocity; andmeans, including a beam con fining structure substantially permeablethroughout its length to said electric signal field but substantiallyimpermeable to unidirectional electric fields and disposed interjacentsaid Wave-transmission line and said reference path, for confining saidbeam to said path while permitting substantially uniform electrostaticcoupling between said line and said beam for signal frequency electricfields but shielding said line therefrom for unidirectional electricfields.

10. An electron-discharge device of the traveling-wave type'cornprising:an electron gun for projecting a sheetlike beam of electrons along apredetermined reference path; a pair of helical wave-transmission lines,individually disposed adjacent opposite sides of said refer,- ence pathin spaced relation thereto and electrostatically coupled to said beam,having a `uniform low wave-propagation velocity in a direction parallelto said path and further having a length in said direction which islarge relative to the effective wave length of a radiofrequency signalwave traveling along said line; means for applying a radio-frequencysignal to said pair of wave-transmission lines in push-pull relationshipto establish said traveling signallwave and thereby to provide atraveling transverse electric signal field which propagates along saidreference path at said low wave-propagation velocity; and means,including a pair of beamconfining structures individually disposedinten'acent respective ones of said pair of wavetransmission lines andsaid reference path, for confining said beam to said path whilepermitting substantially uniform electrostaticY coupling between saidlines and said beam for signal frequency electric fields but shieldingsaid lines therefrom for unidirectional electric fields.

11. An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheet-4 like beam of electrons along apredetermined reference path; a pair of helical wave-transmission lines,indi-4 vidually disposed adjacent opposite sides of said refer,- encepath in spaced relation thereto and electrostatically coupled to saidbeam, having a uniform low wave-propagation velocity in a directionparallel to said path and further having a length in said directionwhich is large relative to the effective wave length of aradio-frequency signal wave traveling along said line; means forapplying a radio-frequency signal to said pair of wave-transmissionlines in push-pull relationship to establish said traveling signal waveand thereby to provide a traveling transverse electric signal fieldwhich propagates along said reference path at said low wave-propagationvelocity; and means including a pairof beam confining structures, eachcomprising two interleaved series of periodically spaced lenselectrodes, individually disposed interjacent respectivel ones of saidpair of wave-transmission lines and said reference path for confiningsaid beam to said path while permitting substantially uniformelectrostatic coupling between said lines and said beam for signalfrequency electric fields but shielding said lines therefrom forunidirectional electric fields.

12. An electron-discharge device of the traveling-wave type comprising:an electron gun for projecting a sheetlike beam of electrons along apredetermined reference path; a pair of helical wave-transmission lines,individually disposed adjacent opposite sides of said reference path inspaced relation thereto and electrostatically coupled to said beam,having a uniform low wave-propagation velocity in a direction parallelto said path and further having a length in said direction which islarge relative to the effective wave length of a radio-frequency signalwave traveling along said lines; means for applying a radio-frequencysignal to said pair of wave-transmission lines in push-pull relationshipto establish said traveling signal wave and thereby to provide atraveling transverse electric signal field which propagates along saidreference path at said low wave-propagation velocity; and meansincluding a pair of beam confining structures, each comprising twointerleaved series of periodically spaced lens electrodes, individuallydisposed interjacent respective ones of said pair of wave-transmissionlines and said reference path with each series of lens electrodes ofeach structure aligned with the corresponding series of the other ofsaid structures and further including means for maintaining each of saidseries of lens electrodes of one of said structures at an averageunidirectional potential equal to the average unidirec- 1 tionalpotential of the corresponding series of lens electrodes of the other ofsaid structures but of an average unidirectional potential substantiallydifferent from that of the remaining series of said electrodes toestablish a space-periodic electrostatic lens for confining saidelectron beam to said path while permitting substantially uniformelectrostatic coupling between said lines and said beam for signalfrequency electric fields but shielding said lines therefrom forunidirectional electric fields.

References Cited in the file of this patent UNITED STATES PATENTS2,183,398 Hehlgans Dec. 12, 1939 2,410,863 Broadway et al. Nov. 12, 19462,489,082 De Forest Nov. 22, 1949 2,509,374 Sunstein May 30, 19502,653,270 Kompfner Sept. 22, 1953 2,683,256 Kumpfer July 6, 1954

